PLA Processing Guide for Spinning Fibers

Transcription

1 PLA Processing Guide for Spinning Fibers This information is intended for use only as a guide for the manufacture of PLA fibers. Because melt spinning and downstream processing of PLA fibers is complex, an experimental approach may be required to achieve desired results. 1.0 Safety and Handling Precautions All safety precautions normally followed in the handling and processing of melted thermoplastics should be followed for NatureWorks PLA resins. As with most thermoplastics, melt processing and the variability of those conditions may result in minor decomposition. Lactide, a non-hazardous gaseous irritant, is a minor by-product of PLA melt processing. Appropriate air testing should be completed to ensure an acceptable Threshold Limit Value (TLV) of less than 5 mg/m 3 is maintained. The use of process area point source remediation measures such as monomer fume hoods or exhausts near the spinneret are typically recommended. PLA is considered non-hazardous according to DOT shipping regulations. When handling PLA resin at room temperature avoid direct skin and eye contact along with conditions that promote dust formation. For further information, consult the appropriate MSDS for the PLA grade being processed. 2.0 Pellet Storage and Blending Recommendation PLA resins should be stored in an environment designed to minimize moisture uptake. Product should also be stored in a cool place at temperatures below 50 C (122 o F). Product that is delivered in cartons or super sacks should be kept sealed until ready for loading into the blending and/or drying system. Bulk resin stored in silos, hoppers etc for extended periods (more than 6 hrs) should be kept purged with dry air or nitrogen to minimize moisture gain. In the case of outside storage, if the product is supplied in Boxes or other non-bulk containers, the unopened container should be brought into the fiber production area and allowed to equilibrate for a minimum of 24 hours before opening. During chip transfer, minimal transfer air temperature (~40C) and velocity (~25 m/s for dilute phase transfer systems) are recommended to minimize the potential of generating fines. 3.0 Resin Properties 6201D is the recommended PLA resin grade for staple fibers. Typical properties of 6201D are shown in the table below. Resin Property Nominal Value RV Melt Temperature ( o C) Glass Transition Temperature ( o C) Crystallization Temperature ( o C)

2 Typical PLA Resin Properties The graph on the following page shows the melt density of PLA over a wide temperature range. PLA M elt Density Curve Melt Density (g/cc) Temp (oc)

3 The rheology curves are shown in the following graph. Shear Viscosity, Pa.s D Rheology, 240C 6200D, 2.8 RV 6200D, 3.1 RV 6200D, 3.4 RV Shear Rate, /sec. 4.0 Materials of Construction All metal parts in the extrusion process should be constructed of stainless steel to minimize corrosion. Furthermore, PLA should not be left in the extruder, polymer filter, polymer transfer lines, spin beam, spinnerets or any other part of the extrusion system at PLA melt temperatures or higher for extended periods. Below is a guideline for the recommended types of steel that should be used in the extrusion system. Part Melt pumps and bearings Pump blocks Transfer lines and spin beam Steel Type SUS440B SUS631 SUS440C 5.0 Drying PLA resin can be successfully dried using most standard drying systems. Recommended conditions are provided for standard desiccant based column dryers. For other drying system designs, additional information can be provided upon request. To prevent equipment corrosion, it is not recommended to dry or store hot PLA resin in carbon steel vessels (see Section 2.0).

4 In-line drying is essential for PLA resins. A moisture content of less than (50 PPM) is recommended to prevent viscosity degradation. Material is supplied in foil-lined containers dried to less than 400 PPM as measured by NatureWorks internal method. The resin should not be exposed to atmospheric conditions after drying. Keep the package sealed until ready to use and promptly dry and reseal any unused material. The drying table below can be used to estimate the drying time needed for PLA. Air or nitrogen based desiccant drying systems can be used at the recommended temperatures. Typical PLA drying conditions are shown in the table below. Typical Settings Drying Parameter Amorphous Crystalline Residence Time (hours) 4 2 Air Temperature ( o C) Air Dew Point ( o C) Air Flow Rate (m 3 /min/kg resin) > > Typical PLA Raw Material Drying Conditions Typical desiccant dryer regeneration temperatures exceed the melt point of PLA resins. To prevent issues with pellet bridging, sticking or melting, the drying system should be verified to ensure temperature control is adequate during operation as well as during regeneration cycles since valve leakage is common in many systems. 6.0 Melt Spinning Prior to introducing PLA into any melt spinning system, the system should be properly purged to prevent any polymer contamination and spinnability problems from occurring. The purging procedures below are recommended for optimal removal of other polymers. 6.1 PLA Purging Procedure 1. Purge with a low MI (<1) PP at normal PET (280 o C) operating temperatures without spinneret in place. Purge for at least 30 minutes, until polymer appears free of contamination. Let system empty as well as possible. 2. Reduce extrusion temperature settings to normal PLA operating temperature ( o C). 3. Begin purging with an 18 MI PP without spinneret in place while temperature is dropping. 4. After temperature targets are achieved, purge for at least 30 minutes, until polymer appears free of contamination. Let system empty as well as possible. 5. Transition to PLA and purge for at least 30 minutes, until polymer appears free of contamination. 6. Insert pre-heated spinneret and allow temperature to equilibrate. 7. Purge with 6201D and evaluate flow from capillaries. As long as flow is even from each capillary and there is no evidence of contamination, begin spinning. Important Notes: 1. It is critical that all drying and conveying/receiving systems be free of all PET or PP and is vacuumed to ensure that there is no remaining polymer dust, before adding PLA. PET will not melt at PLA operating temperatures and will block screens, if it is present in the system 2. Brand of PP used for purging is unimportant, as long as it does not thermally cross-link. 6.2 Extrusion A general-purpose single-screw extruder, 24 to 36:1 L/D with feed-throat cooling is acceptable for processing PLA. A mixing tip is generally recommended along with static mixers in the product line to ensure temperature uniformity as well as optimum additive dispersion and melt polymer homogeneity. The following table shows a typical melt profile for PLA.

5 Extrusion Area Melt Temperature Setting ( o C) Feed throat 25 Zone Zone Zone Melt pump 235 Spin head 235 Typical PLA Extrusion Conditions Note 1: Temperatures are only starting points and may need to be altered. Target PLA melt temperatures (after melt pump) should be in the range of 235±5 C Note 2: PLA resins should not be processed at temperatures above 250 C (482 F) due to excessive thermal degradation. 6.3 Additives Delusterants such as TiO2 are best added as a masterbatch at wt% in PLA resins and controlled dosing the required amount of dried masterbatch into the feed throat of the running extruder. 6.4 Filtration PLA resin will be provided pre-filtered to a level of 20 microns. The following pack makeup is recommended: Loose media (optional - depending upon pack configuration) micron shattered metal is recommended for an uncompressed pack cavity fill. Screens cascade configuration with appropriate support screens is recommended with finest filtration level of 20 microns. 6.5 Heating Systems To allow for the required temperatures to be obtained in spinning, typically vapor heat transfer system medium changes are required. Dowtherm J / Therminol LT or a comparable vapor HTM which has an atmospheric boiling point of 200 C or less while remaining within specific system pressure design limits is generally recommended. Operation of the HTM system at a temperature as close as possible to the actual melt temperature (235±5 C) is recommended to provide an adiabatic spinning system. For vacuum assisted systems, typically heat transfer medium changes are not required as long as the system vacuum can be operated at a level to provide vaporization and uniform heating at the suggested temperatures ( C). Prior to being placed into service, spin packs should be heated to 250 o C to allow for some temperature loss during spin pack installation.

6 6.6 Spinnerets Recommended capillary dimensions for a fiber with a solid round cross section range from mm diameter, typically with a 2 to 3:1 L/D ratio. Larger capillaries may be necessary for fiber s greater than 6 dpf. The following guide can be used to estimate spinneret requirements based on spun product dpf: Spinneret Capillary Sizing 0.4 diameter mm L/D 3 L/D spun dpf Capillary dimensions for modified cross sections will deviate greatly from solid round fibers. They should be designed to meet the desired fiber shape, while providing adequate pressure drop to ensure good denier uniformity and adequate draw down or stretch ratio to facilitate good spinning performance. 6.7 Quenching Filaments should be quenched with air at controlled temperatures and velocities to ensure good denier and orientation uniformity. Typical quenching conditions are shown below, but quenching conditions need to be optimized by product depending upon the denier, spinneret design, and cross section. Quench parameter Typical Target Quench Air velocity (m/s) Quench Air Temperature ( o C) Monomer Exhaust Velocity (m/s) Spacer or Shroud Length (mm) Typical PLA Quenching Conditions A monomer exhaust system is preferred to prevent the buildup of residual lactide around the spinneret face and quench screen. 6.8 Take Up PLA can be spun over a wide range of take up speeds, but typically runs between 1100 and 1850 m/min. 7.0 Drawing A modern polyester type draw line equipped with accurate speed, temperature and tension controls is recommended for drawing PLA. Two-stage drawing is preferred to maximize tensile properties without stress whitening PLA. Fleissner and Neumag manufacture suitable draw lines for PLA.

7 When running on older lines, draw ratios that can be achieved will depend on the degree of control over temperatures, both water and steam, the type of draw roll heating employed, the length of water baths and steam chests and the line speed. In short, uniform drawing depends on heat transfer rates from the equipment to the fibre and the degree of control over drawing temperatures and conditions. Several factors impact the heat transfer rate, including: Tow mass Initial tow temperature. Line speed Contact area of the rolls (diameter) and layout Number of rolls Method of heat application (immersion bath, sparge systems) 7.1 Creeling and pre-tensioning Creel Size Creel size is largely dependent upon the crimper size and the tension rating of the drawstands. PLA can be run at similar tow densities in crimping as PET, with typical densities ranging from 65 to 72 thousand dtex per cm of crimper width Creel Tensioning Creels should be designed to provide a minimal tension level on each subtow in the creel. If the tension is too great, typically greater than 0.2 g/den, the tow could begin drawing before or during immersion in the pre draw bath, causing excessive denier variation along with the possibility of broken fibers and high wrap rates. In addition, the tension level between individual subtows in the creel should be uniform. An adjustable pre tension stand is preferred prior to the pre draw bath to assist in equalising the tension between subtows. 7.2 Pre Draw Bath Pre-draw baths should be sufficiently long to enable saturation of the tow with moisture and finish and to initially raise the tow temperature to o C. The tow should not be heated above PLA s Tg (58 o C), other wise the tow could begin drawing, which would lead to the same types of problems described in the preceding section. Also nip rolls are recommended on the pre draw rolls to minimize tension on the towband before drawing. 7.3 Drawing Temperatures The initial drawing occurs in the first draw stage, where the tow temperature should be maintained between 45 and 70 o C. Roll cooling is recommended on the post draw stand to prevent the tow band from sticking to the rolls. The recommended tow temperature for the second draw stage is o C. Failure to reach recommended temperatures in stage-2 draw zone would result in fiber with an excessive amount of shrinkage once the fiber is heatset under relaxed conditions. To achieve the recommended tow temperatures, heating systems between the draw are required. Where sparge type heating systems are employed, the volumetric flow rate, distribution and temperature uniformity of the steam or water are critical to ensuring uniform tension and drawing across the towband. Inadequate flow and/or poor distribution will lead to poor tow temperature uniformity, resulting in non uniform drawing and potentially broken fiber and/or undrawns. Immersion baths also provide good tension and drawing uniformity, provided that they have good temperature control and adequate circulation capabilities.

8 7.3.2 Draw Ratios PLA can be drawn over a wide range of draw ratios. The optimal draw ratio is dependent upon the type of polymer used, the as spun orientation level, and the desired tensile properties for the product. PLA staple fiber is typically drawn in the 3:1 4:1 range. As mentioned earlier, two-stage drawing is recommended due to potential stress whitening concerns with single stage drawing. If stage 1 draw ratio is too high, the fiber will stress whiten, or craze. The % of the total draw ratio is typically split 70/30 between stage 1 and 2, but will require optimization by process and manufacturing line. 7.4 Finish application Finish should be applied before entering the dryer or heat setter. Applying finish after the dryer will de-stabilize the tensile properties of the fiber with time. Finish should be selected based on planned downstream processing. Goulston Technologies, Inc or Takemoto Oil and Fat Company LTD can recommend and provide finishes for PLA that have been proven for a variety of applications. Depending on subsequent processing and finish types, application levels range from 0.35% up to 0.8% finish on fiber. 7.5 Crimping Lamination, and Presentation Tow density, lamination and presentation upon entry into the crimper are critical to providing uniform crimp across the width of the tow band. Tow stackers and/or ply bars are recommended for achieving uniform tow density across the tow band. Dancer rolls are also recommended to maintain a constant tension on the tow entering the crimper Heating systems Heating capabilities, preferably steam, are recommended prior to and during the crimper process. Pre heating the tow before entering the crimper allows the fiber to bend easily with minimal mechanical pressure. Heating the fiber during crimping provides greater crimp permanency. Typical tow temperatures before and during crimping are shown in the table below. These temperatures may need to be optimized for each process. Typical Temperatures Process location o F o C Entering Crimper In the Crimper Crimping Typical Tow Temperatures in crimping Minimal mechanical pressure should be used to crimp PLA to decrease the potential of fiber damage. The primary process variable for increasing crimp level should be the tow temperature entering the crimper. The flapper pressure should be used as a secondary control parameter to increase crimp level. Typical flapper and roll pressures for a variety of processes are shown below. Flapper and roll pressures will need to be adjusted for each process to achieve desired crimp levels.

9 Product denier Typical flapper pressure (bar) Typical roll pressure (bar) hollow (PLA/PET bico) hollow Heatsetting Typical mechanical pressures in crimping Hot through air dryers are recommended for drying and heatsetting. The dryers should have multiple zones with a cooling zone at the dryer exit. Temperature control is critical, as the objective is to expose the fiber to temperatures as close to the melt point as possible without actually melting the fiber. Recommended drying temperatures for PLA range from 120 to o C. 7.7 Cutting The temperature of the tow should be maintained below 50 o C to ensure that the crimp does not get pulled out due to the tension at cutting. Tension stands are recommended prior to cutting. Conventional rotary cutters are acceptable. 7.8 Baling Conventional polyester balers are acceptable for baling PLA. The bale density required to produce stable bales will vary with denier and finish type. Bale densities range from g/cm 3 (17 25 lb/ft 3 ). Typical pressures applied in baling range from 50 to 100 psi. Low melt point binder fibers require lower ram pressure to minimize the potential of fiber sticking together in the bale. 7.9 Fiber Storage Baled fiber should be stored in cool, clean, dry location to prevent hydrolysis and contamination. Environmental conditions of 25 o C, 55% RH are recommended.

10 8.0 Fiber Properties The table below shows some typical PLA fiber properties. Product Description Fiber Property 1.3 dpf 3 dpf 4 dpf hollow 4 dpf bico 6 dpf 7 dpf hollow 17 dpf Denier CPI Tenacity (g/den) >3.0 >3.0 >3.0 >2.5 >3.0 >2.5 >2.0 Elongation to break (%) Finish on Fiber (%) Hot air shrinkage ( o C, 10 mins) < 6.0 < 6.0 < 4.0 < 15%* < 6.0 < 4.0 < 6.0 * Note: Air temperature of 110C for 10 minutes 9.0 Processing Conditions The conditions below are nominal and may need to be adjusted depending upon individual converters equipment. Detailed Process Conditions for PLA Staple Products Product Description 7 dpf hollow dry 7dpf hollow Slick 1.5 dpf (apparel) 1.5 dpf (nonwovens) Process Specifications Spinning Conditions Cross section Hollow Hollow Solid, Round Solid, Round Polymer PLA PLA PLA PLA Melt Temp. (C) Quench Air Temp. (C) Quench Air velocity (m/s) <0.1 <0.1 Auxiliary Quench Air Velocity (fpm) <0.1 <0.1 Quench Spacer Length (mm) % Void N/A N/A

11 Spin Finish FOY (%) Spin Finish Type PL801 PL801 PL801 PL801 Spin Finish Emulsion Level (%) As Spun dpf Throughput / hole (lbs/hr) Take Up Speed Drawing & Crimping Conditions Pre-Draw Bath Temp. (F/C) 104/40 104/40 104/40 104/40 Draw Stand One Temp. (F/C) 145/63 145/63 135/57 135/57 Stage 1 Drawing Temp. (F/C) 165/74 165/74 165/74 165/74 Draw Stand Two Temp. (F/C) 167/75 167/75 165/74 165/74 Stage One Draw Stage 2 Drawing Temp ( o C) 212/ / / /100 Draw Stand Three Temp. (F/C) 100/40 100/40 100/40 100/40 Line Speed m/min Stage Two Draw Total Draw Ratio Annealing conditions Roll Temperatures Pre or Post Crimper Finish Type ID % Solids in Finish ID % Solids in Finish ID % Solids in Finish ID % Solids in Finish PL PL PL PL If finish is applied post crimper, then % solids PL PL PL PL can be reduced. Also, PL863 and PL801 are PL not necessary in the silicon finish if applied post crimper. PL Total E Level Kilodenier/Inch of crimper width Temperature Entering Crimper ( o F/C) Temperature exiting Crimper ( o F/C) /54 /54 /54 /54 180/82 180/82 180/82 180/82 Roll Pressure BAR Flapper Pressure BAR Oven Residence Time (min)

12 Oven Temp. (C) Zone 1 Zone 2 Zone 3 Zone 4 Post Dryer Finish Type PL811 Post Dryer E Level 15 Post Dryer FOY (%) Si / 0.15 antistat Detailed Process Conditions for PLA Staple Products Product Description 3 dpf dry 4 dpf hollow Slick 17 dpf dry 17 dpf Slick Process Specifications Spinning Conditions Cross section Solid, Round Hollow Solid, Round Solid, Round Polymer PLA PLA PLA PLA Melt Temp. (C) Quench Air Temp. (C) Quench Air velocity (m/s) <0.1 Auxiliary Quench Air Velocity (fpm) <0.1 Quench Spacer Length (mm) % Void N/A N/A N/A Spin Finish FOY (%) Spin Finish Type PL801 PL801 PL801 PL801 Spin Finish Emulsion Level (%) As Spun dpf Throughput / hole (lbs/hr) Take Up Speed Drawing & Crimping Conditions Pre-Draw Bath Temp. (F/C) 104/40 104/40 104/40 104/40 Draw Stand One Temp. (F/C) 145/63 145/63 135/57 135/57 Stage 1 Drawing Temp. (F/C) 165/74 165/74 165/74 165/74 Draw Stand Two Temp. (F/C) 167/75 167/75 165/74 165/74 Stage One Draw

13 Stage 2 Drawing Temp ( o C) 212/ / / /100 Draw Stand Three Temp. (F/C) 100/40 100/40 100/40 100/40 Line Speed m/min Stage Two Draw Total Draw Ratio Annealing conditions Roll Temperatures Pre or Post Crimper Finish Type ID % Solids in ID % Solids in ID % Solids in ID % Solids in Finish Finish Finish Finish PL PL PL PL If finish is applied post crimper, then % solids PL PL PL PL can be reduced. Also, PL863 and PL801 are PL PL not necessary in the silicon finish if applied post crimper. PL PL Total E Level Kilodenier/Inch of crimper width Temperature Entering Crimper ( o F/C) Temperature exiting Crimper ( o F/C) /54 /55 /55 /55 180/82 180/80 180/80 180/80 Roll Pressure BAR Flapper Pressure BAR Oven Residence Time (min) Oven Temp. (C) Zone 1 Zone 2 Zone 3 Zone Post Dryer Finish Type PL811 PL811 Post Dryer E Level Post Dryer FOY (%) Si / 0.15 antistat Si / 0.15 antistat

14 Safety and Handling Considerations Material Safety Data (MSD) sheets for PLA polymers are available from NatureWorks LLC. MSD sheets are provided to help customers satisfy their own handling, safety, and disposal needs, and those that may be required by locally applicable health and safety regulations, such as OSHA (U.S.A.), MAK (Germany), or WHMIS (Canada). MSD sheets are updated regularly; therefore, please request and review the most current MSD sheets before handling or using any product. The following comments apply only to PLA polymers; additives and processing aids used in fabrication and other materials used in finishing steps have their own safe-use profile and must be investigated separately. Hazards and Handling Precautions PLA polymers have a very low degree of toxicity and, under normal conditions of use, should pose no unusual problems from incidental ingestion, or eye and skin contact. However, caution is advised when handling, storing, using, or disposing of these resins, and good housekeeping and controlling of dusts are necessary for safe handling of product. Workers should be protected from the possibility of contact with molten resin during fabrication. Handling and fabrication of resins can result in the generation of vapors and dusts that may cause irritation to eyes and the upper respiratory tract. In dusty atmospheres, use an approved dust respirator. Pellets or beads may present a slipping hazard. Good general ventilation of the polymer processing area is recommended. At temperatures exceeding the polymer melt temperature (typically 170ºC), polymer can release fumes, which may contain fragments of the polymer, creating a potential to irritate eyes and mucous membranes. Good general ventilation should be sufficient for most conditions. Local exhaust ventilation is recommended for melt operations. Use safety glasses if there is a potential for exposure to particles which could cause mechanical injury to the eye. If vapor exposure causes eye discomfort, use a full-face respirator. No other precautions other than clean, body-covering clothing should be needed for handling PLA polymers. Use gloves with insulation for thermal protection when exposure to the melt is localized. Combustibility PLA polymers will burn. Clear to white smoke is produced when product burns. Toxic fumes are released under conditions of incomplete combustion. Do not permit dust to accumulate. Dust layers can be ignited by spontaneous combustion or other ignition sources. When suspended in air, dust can pose an explosion hazard. Firefighters should wear positive-pressure, self-contained breathing apparatuses and full protective equipment. Water or water fog is the preferred extinguishing medium. Foam, alcoholresistant foam, carbon dioxide or dry chemicals may also be used. Soak thoroughly with water to cool and prevent re-ignition. Disposal DO NOT DUMP INTO ANY SEWERS, ON THE GROUND, OR INTO ANY BODY OF WATER. For unused or uncontaminated material, the preferred options include recycling into the process or sending to an industrial composting facility, if available; otherwise, send to an incinerator or other thermal destruction device. For used or contaminated material, the disposal options remain the same, although additional evaluation is required. (For example, in the U.S.A., see 40 CFR, Part 261, Identification and Listing of Hazardous Waste. ) All disposal methods must be in compliance with Federal, State/Provincial, and local laws and regulations. Environmental Concerns Generally speaking, lost pellets are not a problem in the environment except under unusual circumstances when they enter the marine environment. They are benign in terms of their physical environmental impact, but if ingested by waterfowl or aquatic life, they may mechanically cause adverse effects. Spills should be minimized, and they should be cleaned up when they happen. Plastics should not be discarded into the ocean or any other body of water. Product Stewardship NatureWorks LLC has a fundamental duty to all those that make and use our products, and for the environment in which we live. This duty is the basis for our Product Stewardship philosophy, by which we assess the health and environmental information on our products and their intended use, then take appropriate steps to protect the environment and the health of our employees and the public. Customer Notice NatureWorks LLC encourages its customers and potential users of its products to review their applications for such products from the standpoint of human health and environmental quality. To help ensure our products are not used in ways for which they were not intended or tested, our personnel will assist customers in dealing with ecological and product safety considerations. Your sales representative can arrange the proper contacts. NatureWorks LLC literature, including Material Safety Data sheets, should be consulted prior to the use of the company s products. These are available from your NatureWorks LLC representative. NOTICE: No freedom from any patent owned by NatureWorks LLC or others is to be inferred. Because use conditions and applicable laws may differ from one location to another and may change with time, Customer is responsible for determining whether products and the information in this document are appropriate for Customer s use and for ensuring that Customer s workplace and disposal practices are in compliance with applicable laws and other governmental enactments. NatureWorks LLC assumes no obligation or liability for the information in this document. NO WARRANTIES ARE GIVEN; ALL IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR USE ARE EXPRESSLY EXCLUDED. NOTICE REGARDING PROHIBITED USE RESTRICTIONS: NatureWorks does not recommend any of its products, including samples, for use as: Components of, or packaging for, tobacco products; Components of products where the end product is intended for human or animal consumption; In any application that is intended for any internal contact with human body fluids or body tissues; As a critical component in any medical device that supports or sustains human life; In any product that is designed specifically for ingestion or internal use by pregnant women; and in any application designed specifically to promote or interfere with human reproduction. NatureWorks and the NatureWorks logo are trademarks of NatureWorks LLC Copyright 2005 NatureWorks LLC